Cell proliferation inhibitor containing anti-pept antibody

ABSTRACT

The present inventors extensively studied and found that an antibody binding to PepT has cytotoxic activity and inhibits cell growth. These results suggest that an antibody binding to PepT, particularly an antibody having a cytotoxic activity, can be used as a cell growth inhibitor, for example, in treating and preventing cancer.

TECHNICAL FIELD

The present invention relates to an antibody binding to PepT and a cell growth inhibitor containing the antibody as an effective ingredient thereof.

BACKGROUND ART

Mammalian animals need to take in external sources of nutrition and many transport proteins are known to exist in their cells. Many peptide transporters (peptide transport proteins; PepTs) that carry out peptide transport have been found to date (for example, J. Biol. Chem., 270(12):6456-6463, (1995); Biochim. Biophys. Acta., 1235:461-466, (1995); Mol. Microbiol., Vol. 16, p825, (1995); Unexamined Published Japanese Patent Application No. (JP-A) Hei 6-261761; JP-A Hei 11-172; and U.S. Pat. No. 5,849,525). PepT can be classified into proteins that import peptides into cells and proteins that export peptides from cells. They can also be classified according to the different energy sources used during transport. Proton-driven PepTs, which carry out transport by utilizing protein gradient, belong to the PTR family (Mol. Microbiol., Vol. 16, p825, (1995)). PepTs that carry out transport using ATP in the body belong to the ABC family (Annu. Rev. Cell. Biol., Vol. 8, p67, (1992)).

There are reports that PepTs are involved in the transport of not only small-molecule peptides such as dipeptides and tripeptides, but also of pharmaceutical agents such as β-lactam antibiotics and ACE inhibitors (Ganapathy, Leibach., Curr. Biol. 3, 695-701, (1991); Nakashima et al., Biochem. Pharm. 33, 3345-3352, (1984); Friedman, Amidon., Pharm. Res., 6, 1043-1047, (1989); Okano et al., J. Biol. Chem., 261, 14130-14134, (1986); Muranushi et al., Pharm. Res., 6, 308-312, (1989); Friedman, Amidon., J. Control. Rel., 13, 141-146, (1990)).

PepT1 and PepT2 are proton-driven PepTs that contribute to the absorption of proteins and the maintenance of peptidic nitrogen sources by uptaking small-molecule peptides into cells. PepT1 and PepT2 are 12-transmembrane proteins, comprising 708 and 729 amino acids, respectively (J. Biol. Chem., 270(12):6456-6463, (1995); Biochim. Biophys. Acta., 1235:461-466, (1995); and Terada and Inui, Tanpakusitsu Kakusan Kouso., Vol. 46, No. 5, (2001)).

There are reports that PepT1 and PepT2 also transport pharmaceuticals such as β-lactam antibiotics and bestatin (Saito, H. et al., J. Pharmacol. Exp. Ther., 275, 1631-1637, (1995); Saito, H. et al., Biochim. Biophys. Acta., 1280, 173-177, (1996); and Terada, T. et al., J. Pharmacol. Exp. Ther., 281, 1415-1421 (1997)).

PepT1 is mainly expressed in the small intestine and its expression has been confirmed in the kidney and pancreas. Expression of PepT2 has been confirmed in the kidney, brain, lung, and spleen. PepT1 and PepT2 have been reported to be localized in the brush border membrane of intestinal and renal epithelial cells (Ogihara, H. et al., Biochem. Biophys. Res. Commun. 220, 848-852, (1996); Takahashi, K. et al., J. Pharmacol. Exp. Ther., 286, 1037-1042 (1998); Hong, S. et al., Am. J. Physiol. Renal. Physiol., 276, F658-F665 (1999); and Terada and Inui, Tanpakusitsu Kakusan Kouso., Vol. 46, No. 5, (2001)).

Furthermore, overexpression of PepT1 in the cell membrane of human pancreatic duct carcinoma cell lines (Cancer Res., 58, 519-525, (1998)) and the expression of PepT2 mRNA in human pancreatic duct carcinoma cell lines (Millennium World Congress of Pharmaceutical Sciences, (2000)) have been reported. However, the involvement of PepT1 and PepT2 in cancer cell growth was unclear and no discussion had been made as to whether PepT1 and PepT2 when used as target antigens against antibodies will affect cancer cell proliferation.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above observations, aiming at providing an antibody binding to PepT and effectively inhibiting cell growth. Furthermore, this invention also aims at providing a cell growth inhibitor that contains the antibody as an effective ingredient.

The present inventors extensively studied and found that an antibody binding to PepT has cytotoxic activity and inhibits cell growth. These results suggest that an antibody binding to PepT, particularly an antibody having cytotoxic activity, can be used as a cell growth inhibitor.

Specifically, the present invention provides:

-   -   [1] a cell growth inhibitor comprising an antibody binding to         PepT as an effective ingredient;     -   [2] the cell growth inhibitor according to [1], wherein the         antibody binding to PepT has a cytotoxic activity;     -   [3] the cell growth inhibitor according to [2], wherein the         cytotoxic activity is an antibody-dependent cell-mediated         cytotoxic (ADCC) activity;     -   [4] the cell growth inhibitor according to [2], wherein the         cytotoxic activity is a complement-dependent cytotoxic (CDC)         activity;     -   [5] the cell growth inhibitor according to any one of [1] to         [4], wherein the PepT is PepT1;     -   [6] the cell growth inhibitor according to any one of [1] to         [5], wherein the cell growth inhibitor inhibits the growth of a         cancer cell;     -   [7] the cell growth inhibitor according to [6], wherein the         cancer cell is a pancreatic cancer cell;     -   [8] a method for causing toxicity to a cell, wherein the method         comprises the step of administering an antibody binding to PepT;     -   [9] an antibody binding to PepT and having a cytotoxic activity;     -   [10] the antibody according to [9], wherein the cytotoxic         activity is an antibody-dependent cell-mediated cytotoxic (ADCC)         activity;     -   [11] the antibody according to [9], wherein the cytotoxic         activity is a complement-dependent cytotoxic (CDC) activity;     -   [12] the antibody according to [9], wherein the antibody         specifically binds to an extracellular region of PepT;     -   [13] the antibody according to [9], wherein the PepT is derived         from human; and     -   [14] the antibody according to any one of [9] to [13], wherein         the PepT is PepT1.

Firstly, the present invention provides a cell growth inhibitor containing an antibody binding to PepT as an effective ingredient.

In this invention, the phrase “containing an antibody binding to PepT as an effective ingredient” means containing an anti-PepT antibody as a major active ingredient, but it is not intended to limit the anti-PepT antibody content.

There is no particular limitation in the type of an antibody contained in the cell growth inhibitor of this invention so long as it is capable of binding to PepT. In one preferred embodiment, the antibody specifically binds to PepT. In another preferred embodiment, the antibody has a cytotoxic activity.

A cytotoxic activity in this invention includes, the antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). In the present invention, the CDC activity means a cytotoxic activity mediated by a complement system. The ADCC activity in the present invention means an activity to cause cytotoxicity to a target cell when a specific antibody binds to a surface antigen of the target cell, following which an Fcγ receptor-containing cell (such as immunocyte) binds to the Fc moiety of the antibody via the Fcγ receptor.

Whether an anti-PepT antibody has either ADCC activity or CDC activity can be determined by methods well known in the art (for example, Current protocols in Immunology, Chapter 7. Immunologic studies in humans, Editor, John E. Coligan et al., John Wiley & Sons, Inc. (1993)).

Specifically, effector cells, complement solution, and target cells are prepared first.

(1) Preparation of Effector Cells

Spleen is excised from a CBA/N mouse or such to isolate spleen cells in RPMI1640 medium (GIBCO). After washing cells with the same medium containing 10% fetal bovine serum (FBS) (HyClone), the cell density is adjusted to 5×10⁶ cells/ml for preparing the effector cells.

(2) Preparation of Complement Solution

Baby Rabbit Complement (CEDARLANE) is diluted 10-fold in a medium containing 10% FBS (GIBCO) to prepare the complement solution.

(3) Preparation of Target Cells

Pancreatic cancer cell line (e.g., AsPc-1 or Capan-2) cells are radiolabeled by incubation with 0.2 mCi ⁵¹Cr-sodium chromate (Amersham Pharmacia Biotech) in DMEM medium containing 10% FBS at 37° C. for 1 h. Then, cells are washed three times with RPMI1640 medium containing 10% FBS, and adjusted to the cell density of 2×10⁵ cells/ml to prepare the target cells.

Then, ADCC or CDC activity is measured. For ADCC activity, the target cells and anti-PepT antibodies are added (50 μl each/well) into a 96-well U-bottomed plate (Beckton Dickinson), and allowed to react on ice for 15 min. After the reaction, effector cells (100 μl) are added to each well, and the plate is incubated in a carbon dioxide gas incubator for 4 h. The final concentration of the antibody is set at 0 μg or 10 μg/ml. After incubation, the supernatant (100 μl) is collected and the radioactivity is measured via a gamma counter (COBRAIIAUTO-GMMA, MODEL D5005, Packard Instrument Company). Cytotoxic activity (%) can be calculated by the formula: (A−C)/(B−C)×100 wherein A represents the radioactivity (cpm) of each sample; B represents the radioactivity of a sample comprising 1% NP-40 (Nacalai); and C represents the radioactivity of a sample comprising only the target cells.

On the other hand, for CDC activity, the target cells and anti-PepT antibodies are added (50 μl each/well) into a 96-well flat-bottomed plate (Becton Dickinson), and allowed to react on ice for 15 min. Then, the complement solution (100 μl) is added to each well, and incubated in a carbon dioxide gas incubator for 4 h. The final concentration of the antibody is set at 0 μg or 3 μg/ml. After the incubation, the supernatant (100 μl) is recovered to be measured for its radioactivity with a gamma counter. The cytotoxic activity can be calculated in the same manner as the ADCC activity assay.

There are no particular limitations on the antibodies comprised by the cell growth inhibitors of the present invention, as long as they bind to the antigen. Mouse antibodies, rat antibodies, rabbit antibodies, sheep antibodies, chimeric antibodies, humanized antibodies, and human antibodies may be used appropriately. Although the antibodies may be either polyclonal or monoclonal antibodies, monoclonal antibodies are preferred from the point of view that they can stably produce homogeneous antibodies. Polyclonal and monoclonal antibodies can be prepared by methods well known to those skilled in the art.

Hybridoma cells that produce monoclonal antibodies can basically be produced using conventional techniques, described as follows: Specifically, the hybridoma cells can be prepared by (1) conducting immunization using the desired antigen or cells expressing the desired antigen, as the sensitizing antigen according to standard immunization methods; (2) fusing the obtained immunized cells with conventional parent cells by normal cell fusion methods; and (3) screening for monoclonal antibody-producing cells (hybridomas) using normal screening methods.

There is no particular limitation in the type of sensitizing antigen. For example, when PepT is the human PepT1, the human PepT1 protein, cells expressing said human PepT1 protein, partial peptides of the human PepT1 (such as ndltdhnhdgtpds, sspgspvtavtddfkq, tddfkqgqrht, apnhyqvvkdglnqkpe, kdglnqkpekgeng, scpevkvfedisant, and ksnpyfmsgansqkq) and such can be used.

Antigens can be prepared according to methods using baculoviruses (e.g. WO 98/46777).

Hybridomas can be produced according to the method of Milstein et al. (Kohler, G. and Milstein, C., Methods Enzymol. (1981) 73: 3-46). When the antigen has low immunogenicity, immunization can be performed by linking it to a macromolecule with immunogenicity, such as albumin. Recombinant antibodies can also be used, and can be produced by (1) cloning an antibody gene from a hybridoma; (2) incorporating the antibody gene into an appropriate vector; (3) introducing the vector into a host; and (4) producing the recombinant antibodies by genetic engineering techniques (see, for example, Carl, A. K. Borrebaeck, James, W. Larrick, THERAPEUTIC MONOCLONAL ANTIBODIES, Published in the United Kingdom by MACMILLAN PUBLISHERS LTD, 1990). Specifically, cDNAs of the variable regions (V regions) of antibodies are synthesized from hybridoma mRNAs using reverse transcriptase. When DNAs encoding a V region of an antibody of interest are obtained, they are linked to DNAs encoding an antibody constant region (C region) of interest, and are then incorporated into expression vectors. Alternatively, DNAs encoding an antibody V region can be incorporated into expression vectors comprising DNAs of an antibody C region. The DNAs are incorporated into expression vectors such that expression is controlled by expression regulatory regions such as enhancers and promoters. Host cells are then transformed with these expression vectors to express the antibodies.

The anti-PepT antibody of this invention may recognize any epitope existing on the PepT molecule, without being limited to a particular one. However, because PepT is a twelve-transmembrane protein, the epitope present in the extracellular region is preferably recognized.

In the present invention, recombinant antibodies artificially modified to reduce heterologous antigenicity against humans can be used. Examples include chimeric antibodies and humanized antibodies. These modified antibodies can be produced using known methods. A chimeric antibody is an antibody comprising the antibody heavy chain and light chain variable regions of a nonhuman mammal such as a mouse, and the antibody heavy chain and light chain constant regions of a human. A chimeric antibody can be obtained by (1) ligating the DNA encoding a variable region of a mouse antibody to the DNA encoding a constant region of a human antibody; (2) incorporating them into an expression vector; and (3) introducing the vector into a host for production of the antibody.

A humanized antibody, which is also called a reshaped human antibody, is obtained by transplanting a complementarity determining region (CDR) of an antibody of a nonhuman mammal such as a mouse, into the CDR of a human antibody. Conventional genetic recombination techniques for the preparation of such antibodies are known. Specifically, a DNA sequence designed to ligate a CDR of a mouse antibody with the framework regions (FRs) of a human antibody is synthesized by PCR, using several oligonucleotides constructed to comprise overlapping portions at their ends. A humanized antibody can be obtained by (1) ligating the resulting DNA to a DNA that encodes a human antibody constant region; (2) incorporating this into an expression vector; and (3) transfecting the vector into a host to produce the antibody (see, European Patent Application No. EP 239,400, and International Patent Application No. WO 96/02576). Human antibody FRs that are ligated via the CDR are selected where the CDR forms a favorable antigen-binding site. As necessary, amino acids in the framework region of an antibody variable region may be substituted such that the CDR of a reshaped human antibody forms an appropriate antigen-binding site (Sato, K. et al., Cancer Res. (1993) 53, 851-856).

Methods for obtaining human antibodies are also known. For example, desired human antibodies with antigen-binding activity can be obtained by (1) sensitizing human lymphocytes with antigens of interest or cells expressing antigens of interest in vitro; and (2) fusing the sensitized lymphocytes with human myeloma cells such as U266 (see Examined Published Japanese Patent Application No. (JP-B) Hei 1-59878). Alternatively, the desired human antibody can also be obtained by using the desired antigen to immunize a transgenic animal that comprises the entire repertoire of human antibody genes (see International Patent Application WO 93/12227, WO 92/03918, WO 94/02602, WO 94/25585, WO 96/34096, and WO 96/33735). Furthermore, techniques to obtain human antibodies by panning with a human antibody library are known. For example, the variable region of a human antibody is expressed as a single chain antibody (scFv) on the surface of a phage using phage display method, and phages that bind to the antigen can be selected. By analyzing the genes of selected phages, the DNA sequences encoding the variable regions of human antibodies that bind to the antigen can be determined. If the DNA sequences of scFvs that bind to the antigen are identified, appropriate expression vectors containing these sequences can be constructed, and human antibodies can be obtained. Such methods are already well known (see WO 92/01047, WO 92/20791, WO 93/06213, WO 93/11236, WO 93/19172, WO 95/01438, and WO 95/15388).

When the antibody genes have been isolated and introduced into an appropriate host, hosts and expression vectors can be used in appropriate combination to produce the antibodies. As eukaryotic host cells, animal cells, plant cells, and fungal cells may be used. Known animal cells include: (1) mammalian cells such as CHO, COS, myeloma, baby hamster kidney (BHK), HeLa, and Vero cells; (2) amphibian cells such as Xenopus oocytes; or (3) insect cells such as sf9, sf21, and Tn5. Known plant cells include cells derived from the Nicotiana genus such as Nicotiana tabacum, which can be callus cultured. Known fungal cells include yeasts such as the Saccharomyces genus, for example Saccharomyces cerevisiae, and filamentous fungi such as the Aspergillus genus, for example Aspergillus niger. Prokaryotic cells can also be used in production systems that utilize bacterial cells. Known bacterial cells include E. coli and Bacillus subtilis. By transferring the antibody genes of interest into these cells using transformation, and then culturing the transformed cells in vitro, the antibodies can be obtained.

Furthermore, the antibody may be an antibody fragment or a modified antibody thereof, as long as it binds to PepT. For example, the antibody fragment may be Fab, F (ab′) 2, Fv, single chain Fv (scFv) in which Fv from H or L chains are ligated by an appropriate linker, or Diabody. More specifically, the antibody fragment is obtained by (1) treating the antibody with enzymes such as papain and pepsin; (2) transferring it into an expression vector; and then (3) expressing it in an appropriate host cell (see, for example, Co, M. S. et al., J. Immunol. (1994) 152, 2968-2976; Better, M. & Horwitz, A. H. Methods in Enzymology (1989) 178, 476-496, Academic Press, Inc.; Plueckthun, A. & Skerra, A. Methods in Enzymology (1989) 178, 476-496, Academic Press, Inc.; Lamoyi, E., Methods in Enzymology (1989) 121, 663-669; and Bird, R. E. et al., TIBTECH (1991) 9, 132-137).

scFv can be obtained by ligating the V regions of the antibody H-chain and L-chain. In the scFv, the V regions of the H chain and L chain are ligated via a linker, and preferably via a peptide linker (Huston, J. S. et al., Proc. Natl. Acad. Sci. U.S.A (1988) 85, 5879-5883). The V regions of the scFv H chain and L chain may be derived from any of the antibodies described herein. The peptide linker used to ligate the V regions may be any single-chain peptide consisting of 12 to 19 residues. DNA encoding scFv can be amplified by PCR using as a template either the whole DNA, or a partial DNA encoding a desired DNA, selected from a DNA encoding the H chain or the V region of the H chain of the above antibody, and a DNA encoding the L chain or the V region of the L chain of the above antibody; and using a primer pair that defines the two ends. Further amplification can be subsequently conducted using the combination of DNA encoding the peptide linker portion, and the primer pair that defines both ends of the DNA to be ligated to the H chain and the L chain respectively. Once DNAs encoding scFvs are constructed, expression vectors containing the DNAs, and hosts transformed by these expression vectors, can be obtained according to conventional methods. Furthermore, scFvs can be obtained according to conventional methods using the resulting hosts. These antibody fragments can be produced in hosts by obtaining genes encoding the antibody fragments and expressing them in a manner similar to that outlined above. Antibodies bound to various types of molecules, such as polyethylene glycol (PEG), may be used as modified antibodies. Furthermore, antibodies may bind to radioisotopes, chemotherapeutics, and cytotoxic substances such as bacteria-derived toxin. In particular, radiolabeled antibodies are useful. Such modified antibodies can be obtained by chemical modifications of the resulting antibodies. Methods for modifying antibodies are already established in the art. The term “antibody” in the present invention also encompasses the above-described antibodies.

Furthermore, the antibody used in the present invention may be a bispecific antibody. The bispecific antibody may, have antigen-binding sites recognizing different epitopes on the PepT molecule, or may have one antigen-binding site recognizing PepT and the other recognizing a cytotoxic substance such as radioactive substance, chemotherapeutic agent, and cell-derived toxin. In this case, it is possible to inhibit the growth of tumor cells by directly applying the cytotoxic substance to the cells expressing PepT to specifically damage them. Bispecific antibodies can be prepared by linking HL pairs of two kinds of antibodies, or obtained by fusing hybridomas that produce different monoclonal antibodies to prepare fused cells generating bispecific antibody. Furthermore, the bispecific antibody can be generated by using genetic engineering techniques.

Antibodies expressed and produced as described above can be purified by conventional methods for purifying normal proteins. Antibodies can be separated and purified by, appropriately selecting and/or combining affinity columns such as a protein A column, or a chromatography column, filtration, ultrafiltration, salt precipitation, dialysis, and such (Antibodies A Laboratory Manual. Ed Harlow, David Lane, Cold Spring Harbor Laboratory, 1988).

Conventional means can be used to measure the antigen-binding activity of the antibodies (Antibodies A Laboratory Manual. Ed Harlow, David Lane, Cold Spring Harbor Laboratory, 1988). For example, enzyme linked immunosorbent assay (ELISA), enzyme immunoassay (EIA), radioimmunoassay (RIA), or fluoroimmunoassay may be used.

Furthermore, PepT-binding antibodies contained in the cell growth inhibitors of this invention are not particularly limited, however are preferably antibodies binding to PepTs which have the transport activity of incorporating peptides into cells using proton motive force. More preferably, they are antibodies binding to PepT1 or PepT2, and most preferably, they are antibodies binding to PepT1.

The nucleotide and amino acid sequences of PepT1 and PepT2 are already known (human PepT1: GenBank XM 007063 (J. Biol. Chem., 270(12):6456-6463, (1995)); human PepT2: GenBank XM 002922 (Biochim. Biophys. Acta., 1235:461-466, (1995)); mouse PepT1: GenBank AF 205540 (Biochim. Biophys. Acta., 1492:145-154 (2000)); and mouse PepT2: GenBank NM 021301 (Biochim. Biophys. Res. Commun., 276:734-741 (2000))).

Furthermore, a preferred antibody binding to PepT of the present invention specifically binds to the extracellular region of PepT. In this invention, the phrase “specific binding to the extracellular region” means that the antibody is able to immunologically distinguish the extracellular region of PepT from other regions. More specifically, the antibody specifically binding to the extracellular region of PepT only binds to the extracellular region but not to the intracellular region and such as well as transmembrane domains. In this invention, a preferred PepT is the human PepT. The human PepT can be not only derived from human but also obtained as a recombinant by expressing the human PepT in the baculoviral expression system. An immunogen used for obtaining antibody which binds specifically to the extracellular region can include, PepT expressed on the membrane such as cytoplasmic and viral membranes, and fragments containing the PepT extracellular region. Furthermore, regardless of the transporter activity, both PepTs with or without the transport activity can be used as immunogens. For PepT with the transporter activity, PepT expressed on the membrane such as cytoplasmic and viral membranes (for example, PepTs expressed on the Ba/F3 cell membrane and baculoviral membrane) can be used. For example, since PepT is known to incorporate glycylsarcosine into cells as a substrate, it is possible to judge whether the PepT has the transport activity or not by contacting it with [¹⁴C]glycylsarcosine to observe the uptake thereof.

There are no particular limitations as to the cells to be targeted by the growth inhibitors, but cancer cells such as pancreatic cancer cells, liver cancer cells, lung cancer cells, esophageal cancer cells, breast cancer cells, and colon cancer cells are preferred, and pancreatic cancer cells are especially preferred. Therefore, the cell growth inhibitors of the present invention can be used for the purpose of treatment and prevention of diseases caused by cell growth, and more specifically of cancers such as pancreatic cancer.

The cell growth inhibitors of the present invention can be administered either orally or parenterally, but are preferably administered parenterally. Specific examples include injections, nasal formulations, pulmonary formulations, and cutaneous formulations. For example, injections can be administered systemically or locally by intravenous injection, intramuscular injection, intraperitoneal injection, or subcutaneous injection. Furthermore, the method of administration can be selected appropriately according to the age and symptoms of the patient. A single dose can be selected, from within the range of 0.0001 mg to 1,000 mg per kg body weight. Alternatively, the dose can be selected, from within the range of 0.001 to 100,000 mg/body for each patient. However, the dose of a therapeutic agent of the present invention is not limited to these examples.

The cell growth inhibitors of the present invention can be formulated according to standard methods (see, for example, Remington's Pharmaceutical Science, latest edition, Mark Publishing Company, Easton, U.S.A), and may comprise pharmaceutically acceptable carriers and additives. Exemplary carriers include surfactant, excipient, coloring agent, flavoring agent, preservative, stabilizer, buffering agents, suspending agents, isotonizing agent, binder, disintegrator, lubricant, fluidity promoter, and corrigent. However, the carriers that may be employed in the present invention are not limited to this list. In fact, other commonly used carriers can be appropriately employed: light anhydrous silicic acid, lactose, crystalline cellulose, mannitol, starch, carmelose calcium, carmelose sodium, hydroxypropylcellulose, hydroxypropylmethylcellulose, polyvinylacetaldiethylaminoacetate, polyvinylpyrrolidone, gelatin, medium chain fatty acid triglyceride, polyoxyethylene hydrogenated castor oil 60, sucrose, carboxymethylcellulose, corn starch, inorganic salt, and so on.

Furthermore, the present invention provides a method for causing cytotoxicity to cells, which comprises the step of administering the antibody binding to PepT. The antibody binding to PepT has been described above as the antibody binding to PepT contained in the cell growth inhibitor of the present invention. The method of this invention can be used for treating and preventing disorders caused by cell growth, particularly cancers such as pancreatic cancer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts graphs showing the results of FACS analyses which examined the reactivity of the PepT1 antibody toward the pancreatic cancer cell lines, AsPC-1 and BxPC-3, expressing PepT1 and PepT2 at high levels, respectively.

FIG. 2 depicts bar graphs showing the results of CDC activity measurements of the PepT1 antibody in the AsPC-1 and BxPC-3 cells. The upper panel shows the CDC activity toward the AsPC-1 cells while the lower panel toward the BxPC-3 cells.

BEST MODE FOR CARRYING OUT THE INVENTION

Herein below, the present invention is described in more detail with reference to Examples.

1. Preparation of Anti-PepT1 Antibody

1-1. Preparation of DEF2A Antibody

Ba/F3 cells expressing the human PepT1 (Ba/F3-PepT1) were washed with PBS and suspended in PBS to a final density of 4×10⁷ cells/ml. This cell suspension (0.25 ml) was intraperitoneally administered to Balb/c mice (female) for immunization. In a similar manner, immunization was repeated at one- to two-week intervals 19 times in total, followed by the twentieth immunization by administering the cell suspension into the tail vein.

Spleen cells were prepared from these mice, and fused to the mouse P3U1 cells by the common method using polyethylene glycol. Resulting cells were seeded in a 96-well plate, and cultured in a medium containing hypoxanthine, aminopterin, and thymidine (HAT medium) to select hybridomas. The culture supernatant was recovered on the ninth day from the cell fusion, and then screened by ELISA using the germinating baculovirus (BV-ELISA) expressing the human PepT1 (PepT1-BV) as an antigen to select for positive wells.

BV-ELISA was performed as follows. That is, PepT1-BV was diluted to be a concentration of 40 μg proteins/ml in PBS, and distributed in a 96-well ELISA plate (Maxisorp: Nunc) at 100 μl/well. This plate was left at standing at 4° C. overnight or more, allowing PepT1-BV to adsorb to the plate. Using this plate, ELISA was performed according to the common method.

Hybridomas were cloned by the limiting dilution method using the culture supernatant from wells judged positive. The culture supernatant of cloned cells was subjected again to BV-ELISA using the PepT1-BV, and the positive clone DEF2A was identified.

DEF2A was cultured in an expanded scale, and the culture supernatant therefrom was examined for the reactivity toward the human pancreatic cancer cell line AsPC-1 by FACS analysis to reveal that the antibody produced by the DEF2A clone specifically reacts with AsPC-1 (FIG. 1).

1-2. Preparation of BPT01-13 Antibody

The priming of gp64 transgenic mice (Japanese Patent Application No. 2002-180351) was performed by subcutaneous injection of a suspension of PepT1-BV corresponding to 1 mg protein and 200 ng of pertussis toxin in PBS. Subsequent immunizations were carried out by subcutaneous injection of a similarly prepared PepT1-BV corresponding to 500 μg protein (containing no pertussis toxin, however). The final immunization was performed by injecting PepT1-BV (baculovirus expressing the human PepT1: Japanese Patent Application No. 2002-180351) corresponding to 250 μg protein into the mouse tail vein. Spleen cells were prepared from this mouse, and fused to the mouse P3U1 cells by the usual method using polyethylene glycol.

Screening was performed by FACS using the BaF/3-pepT1 cells. Furthermore, by FACS using the BaF/3-pepT2 cells, the monoclonal antibody “BPT01-13” specifically binding to PepT1 was established. Finally, FACS was performed with AsPC-1 and BxPC-3 cells to confirm the specific binding to PepT1 on the cancer cells (FIG. 1).

2. CDC Activity Analysis of Anti-PepT1 Antibody

The CDC activity analysis of anti-PepT1 antibody was performed using the PepT1 expression-positive and -negative pancreatic cancer cell lines (AsPC-1 and BxPC-3 cells, respectively).

AsPC-1 cells were cultured in RPMI medium containing 20% FBS, while BxPC-3 cells in RPMI containing 10% FBS. Cells were seeded on a 96-well plate (1E4 cells/well) and cultured for two days. ⁵¹Cr (Amersham Pharmacia, CJS4) (5 μCi/well) was added to the cells and incubated for one hour to label the cells. After the cells were washed with HAV buffer (300 μl/well), 0.2 μg, 2 μg, or 20 μg/ml anti-PepT antibody (BPT01-13 or DEF2A) was added thereto (100 μl/well), and left at standing on ice for 15 min. Then, 100% baby rabbit complement (CEDARLANE, CL3441, Lot. 6213) was added thereto (100 μl/well), and the mixtures were allowed to stand at 37° C. for 90 min. After the centrifugation (1,000 rpm, 5 min, 4° C.), the supernatants (100 μl/well) were recovered to measure radioactivity with a gamma counter (Packard Instrument Company, COBRAIIAUTO-GAMMA, MODEL 505). By the following equation, CDC activity (%) was obtained: CDC activity (%)=(A−C)×100/(B−C) wherein A represents the radioactivity in each well; B represents the mean radioactivity of the well comprising 2% NP-40 aqueous solution (Nonidet P-40, Nacalai Tesque, 252-23, Lot. M7M7690) (100 μl) instead of the complement; and C represents the mean radioactivity of the well comprising HAV buffer (200 μl) with neither antibody nor complement. Tests were performed in triplicates to calculate the CDC activity value and standard error (FIG. 2).

INDUSTRIAL APPLICABILITY

The present inventors have found that antibodies binding to PepT have cytotoxic activity and inhibit cell growth. These antibodies can be used as a cell growth inhibitor, for example, in treating and preventing cancer. 

1. A cell growth inhibitor comprising an antibody binding to PepT as an effective ingredient.
 2. The cell growth inhibitor according to claim 1, wherein the antibody binding to PepT has a cytotoxic activity.
 3. The cell growth inhibitor according to claim 2, wherein the cytotoxic activity is an antibody-dependent cell-mediated cytotoxic (ADCC) activity.
 4. The cell growth inhibitor according to claim 2, wherein the cytotoxic activity is a complement-dependent cytotoxic (CDC) activity.
 5. The cell growth inhibitor according to any one of claim 1, wherein the PepT is PepT1.
 6. The cell growth inhibitor according to claim 1, wherein the cell growth inhibitor inhibits the growth of a cancer cell.
 7. The cell growth inhibitor according to claim 6, wherein the cancer cell is a pancreatic cancer cell.
 8. A method for causing toxicity to a cell, wherein the method comprises the step of administering an antibody binding to PepT.
 9. An antibody binding to PepT and having a cytotoxic activity.
 10. The antibody according to claim 9, wherein the cytotoxic activity is an antibody-dependent cell-mediated cytotoxic (ADCC) activity.
 11. The antibody according to claim 9, wherein the cytotoxic activity is a complement-dependent cytotoxic (CDC) activity.
 12. The antibody according to claim 9, wherein the antibody specifically binds to an extracellular region of PepT.
 13. The antibody according to claim 9, wherein the PepT is derived from human.
 14. The antibody according to claim 9, wherein the PepT is PepT1. 